Traveling grate conveyors conventionally used for transporting a bed of material in indurating apparatus have a plurality of grate bars, or grate plates affixed at their ends to endless chains which engage sprocket drive wheels. Streams of heating gas are forced through the material bed carried on the grate plates and exchange heat with the material particles in a plurality of heat transfer zones, e.g., initial preheating, drying, preburn and cooling zones. One disadvantage of known traveling grate apparatus is the high heat energy loss which occurs when the grate plates and the grate chain return through the atmosphere, and such heat loss has been measured to be 15 percent of the total energy input required for an iron ore pelletizing plant. Another disadvantage of known traveling grate apparatus is that all of the grate plates and the grate chain are exposed to the hottest indurating temperature and therefore must be of expensive high alloy steel which will withstand the hottest temperature. A still further disadvantage of such traveling grate indurating apparatus is that the bed of material is static on the grate plates with the result that the solid-to-gas contact, as well as the heat transfer between the gas stream and the bed material, are relatively low. The depth of the bed is limited to one value in a traveling grate indurating system because the material is static on the grate plates and consequently the depth of bed cannot be varied when the material, e.g., iron ore pellets, hardens and increases in strength. The static nature of the bed limits the maximum bed depth to that magnitude, e.g., seven inches, which will not result in crushing the green pellets in the lower layers, since crushing of the pellets results in undesired fusion of crushed pellets with consequent partial blocking of the heating gas stream as it passes through the bed.
Apparatus for conveying and sizing material bed particles is disclosed in such patents as U.S. Pat. Nos. 2,974,793; 2,988,781; 3,438,491 and 3,848,744 comprising rotatably driven rollers having progressively increased spacing between adjacent rollers to permit different size particles to fall therethrough. U.S. Pat. No. 3,438,491 discloses such conveying and particle classifying apparatus having rollers covered with abrasive resistant rubber to minimize damage to the rollers, and U.S. Pat. No. 3,848,744 discloses such conveying and classifying apparatus for taconite pellets having rollers with a hard chromium outer surface to provide high resistance to abrasion and damage by the pellets. However such particle conveying and classifying apparatus is not capable of heat exchange with the bed material while it is being conveyed.